The present invention relates to a means and method for controlling the air-to-fuel ratio of a combustible mixture fed to a combustion device. The invention is broadly applicable to such purpose and is specifically useful in adjusting the air-to-fuel ratio of the combustible mixture fed to an internal combustion engine or to a catalytic combustor.
The prior art is replete with schemes for controlling air-to-fuel ratios, in particular for controlling the air-to-fuel ratio of the combustible mixture fed to an internal combustion engine. Often, the purpose of controlling the air-to-fuel ratio is to enhance the operation of a catalytic exhaust gas purification device while maintaining acceptable levels of engine performance and efficiency. U.S. Pat. No. 4,202,301, assigned to the assignee of this application, discloses an air-to-fuel ratio control mechanism which operates in partial response to a control circuit which, in turn, is responsive to the output of an oxygen sensor mounted in the exhaust line of the engine. This type of arrangement is well-known in the art, and is often referred to as a "closed-loop" operation in which the carburetor or other air-fuel metering device is regulated by a control means which receives, as an input to it, a signal corresponding to the presence of oxygen in the engine exhaust gas, as sensed by the sensor.
U.S. Pat. No. 4,019,474 discloses apparatus which comprises a detector for detecting the density of one component, for example oxygen, in the exhaust gases from an internal combustion engine for the purpose of regulating the air-to-fuel ratio. The regulation is based on the detected value of the component in such a manner that the air-to-fuel ratio approximates the theoretically correct air-to-fuel ratio, i.e., the stoichiometric ratio.
U.S. Patent 3,986,352 discloses a fuel control for an engine equipped with a catalytic convertor which engine is normally operated "closed-loop" with a feedback signal from an air-fuel ratio sensor in the engine exhaust. As illustrated in the figure of the patent, an exhaust gas sensor produces an output voltage which exhibits a great rate of change in response to composition changes when operation is in the vicinity of the stoichiometric air-fuel ratio. The signal generated by the sensor passes through a control unit and ultimately influences the air-fuel ratio control means of a carburetor. A relatively inexpensive and commercially available zirconia-type oxygen sensor displays this characteristic of showing, in the vicinity of a stoichiometric air-to-fuel ratio, a large change in the voltage of the output signal with change in the air-to-fuel ratio. However, its sensitivity to change in the air-to-fuel ratio diminishes rapidly as the air-to-fuel ratio moves away from stoichiometric, either to the lean or the rich side. For this reason, if it is desired to run either rich or lean a significant distance away from stoichiometric it is not feasible, with a zirconia type oxygen sensor, to control the air-to-fuel ratio at a selected value differing significantly from stoichiometric.
This inherent limitation of oxygen (or other gas component) sensors is not a problem with conventional internal combustion engines, such as automobile engines, which are equipped with the so-called three-way conversion catalysts utilized to purify the exhaust gas. Three-way conversion catalysts of the type disclosed in U.S. Pat. No. 4,157,316, assigned to the assignee of this application, are typically used to treat exhaust gases by substantially simultaneously oxidizing carbon monoxide and unburned hydrocarbons and reducing nitrogen oxides to nitrogen. For most efficient operation of these catalysts, the engine should be operated at or very close to stoichiometric air-to-fuel conditions. Accordingly, a conventional zirconia or other sensor is useful since it is sensitive to changes in the air-fuel ratio range about stoichiometric conditions. Accordingly, the reference signal generated by the sensor can be utilized to make adjustments to the air-to-fuel ratio control of the carburetor or fuel injection device so as to hold operation at or very close to stoichiometric.
However, for certain applications, it is desired to run either on the rich or on the lean side of stoichiometric at an air-to-fuel ratio which is outside the sensitivity or response range of available oxygen or other gas component sensors. For example, it is believed that fuel economy can be increased or maximized under lean air-to-fuel ratio conditions, generally about 1 to 3 air-to-fuel ratios lean of stoichiometric. Operation in this range would preclude the use of a closed loop system using an exhaust gas sensor which is sensitive only within a limited range about stoichiometric. In certain other applications it is desirable to operate on the rich side of stoichiometric, outside the response range of conventional sensors. For example, natural gas fueled internal combustion engines such as the type utilized in pipelines and oil fields, require maintenance of a rich A/F ratio which lies outside the limited range of available, reasonable cost sensors.
While means exist for determining exhaust gas component concentration levels resulting from air-to-fuel ratios which lie outside the conventional sensor response range, such equipment is either very expensive, complicated or delicate, or all three, as compared to sensors of the type described above, and therefore are not practical for utilization in most internal combustion engine or catalytic combustor applications. Whether used in a motor vehicle or in stationary equipment, such as a stationary internal combustion engine or a catalytic combustor, it would be desirable to be able to use the less expensive and relatively rugged sensor in lieu of more complicated, delicate and/or expensive sensors or analytical equipment.
It is accordingly an object of the present invention to provide a method and means for controlling the air-to-fuel ratio at a selected value or values which may, but not necessarily are, outside the responsiveness range of the conventional oxygen or other gas component sensor, while nevertheless employing such a sensor as the exhaust gas sensing means.
It is a further object of the present invention to provide a method and means utilizing an exhaust gas component sensor for controlling the air-to-fuel ratio fed to a combustion device, such as an internal combustion engine or a catalytic combustor, without necessity of rapidly oscillating between rich and lean operation.
It is a further object of the present invention to utilize the signal generated from a conventional oxygen (or other gas component) sensor to operate a combustion device, eg., a natural gas fuel engine, at an air-to-fuel ratio displaced from stoichiometric; eg., rich of stoichiometric, while reducing to a minimum or substantially eliminating "wrong side", eg., to lean of stoichiometric, excursions.
Yet another object of the present invention is to provide a control system which operates "open loop" for a proportion of operating time, i.e., without continuous adjustment of the air-to-fuel ratio.
Other objects and advantages of the invention will become apparent from the following description.